Antireflection coating for an inner surface of cemented lenses

ABSTRACT

An improved arrangement for minimizing reflection of light from the interface between a lens and the adjoining cement has two reflection preventing layers between the lens and the cement. This is of particular value where the difference in index of refraction of the lens and cementing material is greater than about 0.2. The product of the indices of refraction of the two coating layers equals the product of the indices of refraction of the adjacent lens and the cementing material.

O Umted States Patent [191 [1 11 3,809,459 Wakamiya et al. May 7, 1974A'NTIREFLECTION COATING FOR AN 3,706,485 :2/1972 Fawcett 350/164 INNERSURFACE OF CEMENTED LENSES [75] Inventors: Shunichiro Wakamiya, Tokyo;'f Examif'e' Archie Borcheh Kiyoshi ltoh, Fukuoka-machi, both Assam"!Exammer cf of Japan Attorney, Agent, or F1rm-Chnstle, Parker & Hale [73]Assignee: Asahi Kogaku Kogyo Kabushiki Kaisha, Tokyo, Japan [57]ABSTRACT [22] Filed: 13 1972 An improved arrangement for minimizingreflection of light from the interface between a lens and the adjoin- PPN0-1297,356 ing cement has two reflection preventing layers between thelens and the cement. This is of particular [52] U.S. Cl. 350/164 Valuewhere the difference in index of refraction of [51] Int. Cl. G02!) 1/10the lens and cememing material is greater than about [58 Field of Search350/164, 165, 166 921cm? P du of h fls ft fif e l qfl of the two coatinglayers equals the product of the indices of [56] References (medrefraction of the adjacent lens and the cementing ma- UNITED STATESPATENTS 3,533,850 10/1970 Tarneya 350/164 4 Claims, 6 Drawing FiguresPATENTED lm 7:914

SHEU 3 (IF 3 u/ 0 m 7 5 mm 5 7 445 2 1 1 1 M mffl 3 m I m 6 I t m 0 O An (1 2 in m a O a Wave length y) 0-0 mew III 60 5 .5 4 (I1 I n /I III IM m. 5 I E fl mom I 7 5 2 5 M Q. i 1 2 a n m m I d 7 1C0 6 8 M 1 1 A n m2 I 5 u u 1 w u .\\u\ R w 6 H 2 8 2 0 m. m. o m. m m M w 0 o o a o a 0 oo WaVQ Iensth (my) ANTIREFLECTION COATING FOR AN INNER SURFACE OFCEMENTEI) LENSES BACKGROUND Vacuum evaporation of magnesium fluoride(MgF single-layered coating on optical parts such as lenses and prismshas already been well known as a measure for reduction of surfacereflection. FIG. 1 gives a group of curves plotting the spectralreflection factor characteristic in such a case utilizing the refractiveindex of each optical part as a parameter, wherein the abscissaindicates the wave length (m of light and the ordinate indicates thesurface reflection factor Rt percent). The symbol n represents therefractive index of an incident medium (air in this case), n, representsthe refractive index of the MgF, coating, and n, the refractive index ofoptical material such as glass.

FIG. 2 gives a group of curves plotting the reflection factor presentedby one of the boundary surfaces defined between balsam and opticalmaterial utilizing the respective optical material as the parameter, inthe case where optical materials of different refractive indices arecemented one to another with balsam as an optical cementing agent. Thereflection factor is calculated, in this case, using balsam with therefractive index of 1.52 as the incident medium. Comparison ofcorresponding curves in FIGS. 1 and 2 with respect to two opticalmaterials of 1.45 n, I .80 clearly indicates that the average reflectionfactor in FIG. 2 is lower than that in FIG. I. In case of a lens systemsuch as photographic lens which comprises a plurality of unitary lenses,therefore, it has usually been considered that, the larger number ofcementing surfaces, the advantageously higher the transmission factor.

As the working technique of reflection preventing coating has rapidlyprogressed, however, multi-layered reflection preventing coatings havebecome easier, permitting the outer surface reflection factor of opticalparts to be reduced to an extremely low value. FIG. 3 gives a group ofcurves plotting this by way of example utilizing the refractive index ofeach optical material as parameter. From comparison of correspondingcurves in FIGS. 3 and 2, it is shown that the average value of thereflection factor of a cementing surface is sometimes higher dependingon the refractive index of the particular optical material, namely, in arange of n, 1.65. It has become important to minimize internalreflection when the difference in indices of refraction of the opticalmaterial and the cementing material is greater than about 0.2.Furthermore, there are always provided two boundary surfaces betweenbalsam and optical material in case of cemented lenses and the innersurface reflection due to the internal cementing surfaces isconsiderably higher than that of external multi-layered reflectionpreventing coating surface. Although it could be considered even that,in such a case, a lens system free from any cementing surface is ratheradvantageous, employment of these cementing surfaces often cannot beavoided in view of other requirements such as efficiency with whichvarious kinds of aberration are compensated and facility with whichoptical systems of complicated configuration such as an erecting prismis fabricated or worked. There must be provided, therefore, an effectivereflection preventing measure on inner cementing surface of the lenssystem.

An object of the present invention is to reduce the inner surfacereflection of the inner cementing surface to the maximum extent.

BRIEF SUMMARY OF THE INVENTION There is therefore provided in practiceof this invention according to a presently preferred embodiment, animproved low reflection interface between an optical glass and anoptical cementing material wherein the difference between the indices ofrefraction of the glass and cementing material is greater than about 0.2comprising a pair of reflection preventing layers between the glass andthe cementing material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows the light reflectioncharacteristic of single-layered coating of magnesium fluoride;

FIG. 2 shows the reflection factor of balsam-glass boundary surface;

FIG. 3 shows by way of example the reflection characteristic ofmulti-layered reflection preventing coat- 8;

FIG. 4 shows the reflection characteristic of singlelayered coatingadjacent balsam;

FIG. 5 shows by way of example the characteristic of double-layeredreflection preventing coating adjacent air; and

FIG. 6 shows the reflection characteristic of doublelayered coatingadjacent balsam.

DETAILED DESCRIPTION It is well known that, when light is incident uponsingle-layered coating on an optical base plate in the direction normalthereto, the reflection is zero at a particular wave length A whichmeets the amplitude requirement n, n n, and the phase requirement md, MNwhere n, is the refractive index of the optical material used,

n is the refractive index of the coating,

n is the refractive index of the ambient mediumt i.e.,

incident medium),

a is the thickness of the coating, and

It is the observed wavelength.

The indices of refraction are conventionally stated as of the d-line ofsodium or about 587.6 mg.

The ambient medium is air in the general case, and, accordingly, n I.But according to the present invention, this corresponds to therefractive index of the optical cementing agent used.

On the assumption that balsam of n 1.52 is used as the optical cementingagent and A is 5 l Omp. near to the center of the visible range, thereare a group of curves as given by FIG. 4 which plot the spectralreflection characteristic utilizing the refractive index n, of eachoptical material as a parameter. As shown by comparison of the group ofcurves in FIG. 4 with those in FIG. 2, the reflection preventing effectin FIG. 4 is substantially higher than that in FIG. 2. It should benoted here, however, that FIG. 4 concerns the case where reflection isprevented by single-layered coating and reflection is zero only withrespect to a particular wave length. This means that reflection remainsin wave length ranges near the ultraviolet and infrared ranges. It isfound that a multi-layered coating arrangement is required forelimination or reduction of such a residual reflection.

A reflection preventing coating of Muchmore, Journal of The OpticalSociety of America, Vol. 38, I948 pages 20-26 is an extremely effectiveone which is not only the simplest in construction of all multi-layeredreflection preventing coatings but also presents a relatively lowreflection factor over a wide wave length range. Requirements for thecoating of this type are as follows, on the assumption that the coatingis doublelayered and the direction of incidence is normal thereto.

Amplitude requirement: mn mm, Phase requirement:

where n, is the refractive index of the optical material used,

n, is the refractive index of the first layer of the coating (the layeradjacent to the ambient medium), n, is the refractive index of thesecond layer of the coating (the layer adjacent to the opticalmaterial),

n is the refractive index of the ambient medium, d, is the thickness ofthe first layer of the coating the layer adjacent to the ambientmedium), d is the thickness of the second layer of the coating (thelayer adjacent to the optical material),

8, 21m d llt, and

The wavelength A used for 8 herein is the d-line. Reflection is zero atthe points corresponding to the two wave lengths which satisfy therequirements as mentioned just above and it is possible to obtain awider range of low reflection than in case of single-layered coating.Moreover, 8 involved in the phase requirement may be appropriatelyselected to vary the interval between these two wave lengths acrosswhich reflection is zero and it is easily possible to widen the range oflow reflection. FIG. 5 shows by way of example an arangement of thereflection preventing coating which satisfies the requirement withrespect to the case where n, L6 to 1.8 and n I, that is, at an externalsurface.

The reflection preventing coating of this arrangement is of an extremelyadvantageous characteristic, but the refractive index of the coatingobtained from the amplitude requirement is too small to be actuallyrealized. Accordingly, no reflection preventing coating of thisarrangement has ever been realized.

According to the present invention, the ambient medium is not air n l)but the optical cementing agent (n I), so that there is a possibility ofrealizing this reflection preventing coating of Muchmore. Assuming thatthe optical cementing agent, e.g., balsam, has a refractive index n1.52, there are provided a group of curves plotting the spectralreflection characteristic as given by FIG. 6 wherein the refractiveindex of each optical material is utilized as the parameter.

Comparison of these curves in FlG.6 with a group of curves in F IG. 2which plot the reflection characteristic of optical material-balsamboundary surface shows that reflection is sufficiently low in FIG. 6 toprovide an almost perfect reflection preventing coating, even when thedifference in indices of refraction is above about 0.2. Furthermore, therefractive index required here for the coating is one that may be easilyrealized in the present state of the art.

This reflection preventing coating for an inner cementing surface isadvantageous also in color balance compensation of light transmitted bya lens system. Recently glass of various refractive indices has beenused as lens material and glass of higher absorption on the side ofshorter wave lengths also has been used more and more often. As aresult, the lens system comprising a plurality of unitary lensesgenerally has a problem with respect to the color balance in the visiblerange and present the spectral transmission characteristic that there ishigher absorption on the side of shorter wave lengths. This problem hasusually been compensated by an additional coating such as Magenta oramber-coating. Such a measure, however, has increased reflection on eachsurface with a result that the performance of lens has been lowered.

In case of a single-layered coating (refractive index n the intensity ofreflected light never exceeds so long as n; n,. Accordingly, the lessthe difference between the refractive index n of the incident medium andthe refractive index n, of the base glass, the easier is is to maintainthe maximum value of the intensity of reflected light low. This meansthat it is possible to compensate the color balance of lens systemtransmission factor without increasing reflection of inner cementingsurface by appropriate selection of the central wave length A whenbalsam having a refractive index which is relatively near to that ofglass is employed as the incident medium. This is the secondadvantageous aspect of vacuum evaporation of the coating according tothe present invention on the inner cementing surface.

Realization of the reflection preventing coating of such a constructionas described hereinbefore requires the coating to have a refractiveindex that satisfies the respective phase and amplitude requirements.Although there are various methods to obtain the coating of such adesired refractive index including a method in which a mixed coating isobtained by simultaneous vacuum evaporation of several kinds of wellknown substances at an appropriate ratio, a method of reproducing therefractive index of coating which is the easiest at present is analternate super-position of two kinds of coating respectively havinghigher and lower refractive indices. This method can easily realize thecharacteristic which is equivalent to that of a single coating havingthe desired refractive index.

As it will be obviously understood from the aforegoing description, thereflection preventing coating of the present invention provides analmost perfect reflection preventing coating for the inner cementingsurface of a lens system and enables the color balance to be effectivelycompensated without an increase of reflection loss.

What is claimed is:

1. An improved low reflection interface between an optical glass and anoptical cementing agent wherein the difference between the indices ofrefraction of the glass and cementing agent is greater than about 0.2consisting essentially of a pair of reflection preventing layers betweenthe glass and the cementing agent wherein the indices of refractionsatisfy the relation n n n n, where n, is the refractive index of afirst of the pair of layers, n, is the refractive index of the secondpair of layers, n is the refractive index of the cementing agent and n,is the refractive index of the optical glass.

2. An improved cemented optical element comprising a first transparentoptical member having an index of refraction n a second transparentoptical member, and a cementing material cementing the second opticalmember to the first optical member, the cementing material having anindex of refraction n and wherein the differences of indices ofrefraction n and n is greater than about 0.2, and improved reflectionpreventing coating between the first optical member and the secondoptical member consisting essentially of a first coating layer having anindex of refraction n and a second coating layer having a differentindex of refraction n, wherein the product of the indices of refractionof the two coating layers equals the product of the indices ofrefraction of the first optical member and the cementing material.

3. A reflection preventing coating for an inner surface of a cementedlens having an optical material with an index of refraction of n and acementing material having an index of refraction of n comprising:

a first coating layer having an index of refraction of n, and athickness of d, between the optical material and the cementing material;

a second coating layer having an index of refraction of n, and athickness of d, between the optical material and the cementing material;and wherein said indices of refraction and thicknesses satisfying therelations n n =n n and n. d n d A/4, wherein A is in the visiblespectrum.

4. ln a cemented lens having at least a pair of lens elements cementedtogether by an optical cementing material having a sodium d-line indexof refraction n substantially greater than l.0, one of said lenselements having a sodium d-line index of refraction n at least 0.2greater than n the improvement consisting essentially of a Muchmore typeantireflection coating betweenthe cementing material and the cementedlens having a pair of M4 thick coating layers having sodium d-lineindices of refraction of n and n respectively, and

wherein n, n, n n

* i t I!

2. An improved cemented optical element comprising a first transparentoptical member having an index of refraction ng, a second transparentoptical member, and a cementing material cementing the second opticalmember to the first optical member, the cementing material having anindex of refraction n0 and wherein the differences of indices ofrefraction ng and n0 is greater than about 0.2, and improved reflectionpreventing coating between the first optical member and the secondoptical member consisting essentially of a first coating layer having anindex of refraction n1 and a second coating layer having a differentindex of refraction n2 wherein the product of the indices of refractionof the two coating layers equals the product of the indices ofrefraction of the first optical member and the cementing material.
 3. Areflection preventing coating for an inner surface of a cemented lenshaving an optical material with an index of refraction of ng and Acementing material having an index of refraction of n0 comprising: afirst coating layer having an index of refraction of n1 and a thicknessof d1 between the optical material and the cementing material; a secondcoating layer having an index of refraction of n2 and a thickness of d2between the optical material and the cementing material; and whereinsaid indices of refraction and thicknesses satisfying the relations n1n2 n0 ng and n1 d1 n2 d2 lambda /4, wherein lambda is in the visiblespectrum.
 4. In a cemented lens having at least a pair of lens elementscemented together by an optical cementing material having a sodiumd-line index of refraction n0 substantially greater than 1.0, one ofsaid lens elements having a sodium d-line index of refraction ng atleast 0.2 greater than n0, the improvement consisting essentially of aMuchmore type antireflection coating betweenthe cementing material andthe cemented lens having a pair of lambda /4 thick coating layers havingsodium d-line indices of refraction of n1 and n2, respectively, andwherein n1 n2 n0 ng.